It is well-known in the art that cardiac stimulating devices can emit back-up pulses to a heart ventricle when no evoked response to a previously applied ventricular pacing pulse is detected, see U.S. Pat. No. 5,476,487.
This type of cardiac stimulating devices is generally denoted as autocapture pacemakers in the art. The autocapture pacemakers operate by sensing whether a response occurs in the ventricle shortly after a ventricular stimulating pulse has been delivered, such as 5-20 ms after delivery. The sensing may, for instance, take place during 40-80 ms. If a response is sensed during this time interval then no back-up pulse is emitted. However, if no evoked response is sensed, the pacemaker emits a back-up pulse. The back-up pulse is usually emitted with increased output energy or power in order to secure capture of the heart. For example, a normal value for a ventricular pacing pulse could have an amplitude of about 1.5 V, whereas the back-up pulse can be emitted with an amplitude of about 4.5 V.
Sometimes a back-up pulse can be delivered to the ventricle during a phase of the heart cycle when a stimulating pulse is not wanted. Such unwanted pulses may be initiated, for instance, after the occurrence of a premature ventricular contraction (PVC). If such a PVC occurs more or less simultaneously with an atrial stimulating pulse, the PVC may be undersensed due to ventricular blanking after atrial stimulation. In such a case, a ventricular stimulating pulse may be delivered in the PVC refractory period, during which the heart will not respond to ventricular stimulation. No evoked response will consequently be detected and a back-up pulse is delivered. In an unfortunate case, the back-up pulse can be timed with the vulnerable phase of the PVC, typically coincident with a part of the T-wave, during which a delivered stimulating pulse may induce repetitive rhythms, such as for example tachycardia or ventricular fibrillation (VF). A back-up pulse poses somewhat higher threat due to its increased energy content.
A related problem also exists for cardiac stimulating devices not running in the autocapture operation mode. With these cardiac stimulating devices, the delivery of the ventricular stimulating pulse can coincide with the vulnerable phase of a PVC, in particular if a long atrioventricular interval has been programmed for the cardiac stimulating device. Such comparatively long atrioventricular intervals are commonly used within the field in order to avoid unnecessary ventricular stimulation.
U.S. Pat. No. 6,952,609 discloses an implantable cardiac stimulating device capable of combating problems of increased risk of unintentionally triggering tachycardia or VF due to unfortunate timing of back-up pulses. The implantable cardiac stimulating device has a control unit for controlling an atrial pulse generator dependent on an atrial sensing unit, in a first manner wherein no stimulating pulse is delivered to the atrium and in a second manner wherein stimulating pulses are delivered to the atrium. The control unit additionally prevents delivery of back-up pulses to the ventricle during a number of heart cycles when the control unit changes from the first manner of operation to the second manner of operation.
US 2004/0158292 discloses an implantable cardiac stimulating device having an atrial pulse generator capable of generating and delivering an atrial stimulating pulse to an atrium of a heart. The delivered atrial pulse triggers a blanking period, during which a PVC may occur. Atrial and ventricular sense amplifiers connected to the pace/sense electrodes of an atrial lead and a ventricular lead, respectively, are inactive during the blanking period and will not sense the PVC. It is therefore a risk that a subsequently ventricular stimulating pulse will be delivered by the implantable cardiac stimulating device during the vulnerable period of the ventricles caused by the PVC.
The implantable cardiac stimulating device consequently has a dedicated PVC sense amplifier connected to a lead electrode and a can electrode or different can electrodes to perform far field PVC sensing. The PVC sense amplifier is blanked during delivery of the atrial pulse but is then directly activated in order to be able to detect any PVC occurring in the normal blanking period of the ventricular sense amplifier.